Irules changing BigIP configs?

Actually, that's not quite true Deb.

One could disable a pool member from within an iRule with the LB::down LB::up commands added in 9.2. You just need to be aware that the monitors will still continue to think that the pool member is up, though the pool member will not be picked. More work in this area is coming in a subsequent release.

Also, I think you could accomplish what Les wants to do with something like this:

when RULE_INIT {
   set ::active_FTP 10.0.0.1
   set ::backup_FTP 10.0.0.2
}
when CLIENT_ACCEPTED {
   pool FTP_pool member $::active_FTP 21
}
when LB_FAILED {
   if { [LB::status pool FTP_pool member $::backup_FTP 21] eq "up" } {
      set temp $::active_FTP
      set ::active_FTP $::backup_FTP
      set ::backup_FTP $temp
      pool FTP_pool member $::active_FTP 21
   }
}